SAN FRANCISCO — The state Public Utilities Commission on Thursday slapped PG&E with a $1 million fine and ordered a potential $400 million reduction in monthly natural gas bills as punishment for the utility”s attempts to improperly influence the outcome of a rate case linked to a fatal explosion in San Bruno.

Critics slammed the decision, saying it did not go far enough to disrupt the cozy relationship between the utility and state regulators. The ruling was in response to a widening email scandal showing that PG&E and regulators worked closely together to find a judge who would craft a decision favorable to the utility in a key gas transmission and storage rate case.

“The PUC”s credibility could not be lower,” said Thomas Long, legal director with The Utility Reform Network, a consumer group. “The agency should be taking steps to restore that credibility. That”s what needs to happen. The PUC is turning a deaf ear to that need.”

Critics noted that while regulators Thursday ordered PG&E to disclose all its future communications with the five commissioners and top officials such as department heads and their deputies, the ruling also allows the company to conduct private meetings with potentially hundreds of key staffers that it could influence without reporting the contacts.

Investigators believe the deadly San Bruno disaster resulted from a combination of PG&E”s shoddy maintenance, flawed record keeping and the commission”s lax oversight of the utility.

San Francisco-based PG&E vowed to appeal the 3-0 decision.

“We fully understand that we are accountable when we fail to uphold high standards,” PG&E said in a prepared statement. “The PUC decision imposes sanctions that aren”t warranted and that may go beyond the PUC”s legal authority.”

Earlier this year, PG&E voluntarily released dozens of emails that disclosed improper contacts between the company and regulators. The emails showed PG&E attempted to influence a crucial $1.29 billion natural gas transmission and storage rate case that could result in an 11.8 percent increase in monthly residential gas bills. The company forced three executives to resign because of their role in the scandal.

PUC President Michael Peevey and Commissioner Michael Florio recused themselves from the Thursday decision because both regulators were involved in email discussions that showed they sought to accommodate PG&E on multiple issues.

While Thursday”s decision shields ratepayers from the full impact of the cost of the gas system upgrades, it”s unclear how the ruling will ultimately impact residential gas bills. The commission won”t vote until sometime in 2015 on how much revenue PG&E should be allowed to collect from ratepayers, and how much to charge them on monthly bills, for the upgrades and repairs to its gas system.

In December 2013, PG&E had requested about $1.29 billion in higher revenue for improvements to the gas system. The Thursday ruling enables the PUC to deny up to $400 million of that — and will require shareholders to make up the difference. An equivalent one-third reduction in the fee for residential gas customers means that bills might rise just $3.49 a month rather than the currently projected $5.23, although those numbers are fluid.

“Through this decision, we continue to send a signal to PG&E that we expect full compliance and appropriate respect for the PUC”s processes and its staff, as well as fair treatment to its consumers,” said PUC Commissioner Carla Peterman, who crafted Thursday”s decision. “It is my hope that we can move on to a fair and balanced consideration of the appropriate investments that PG&E needs to make in its gas transmission and storage systems moving forward.”

San Bruno officials and TURN have demanded that PG&E release 65,000 emails related to the gas transmission case and other matters.

“The commission had an opportunity to order that all these emails are to be released,” said Britt Strottman, an Oakland attorney who represents San Bruno. “The PUC had an opportunity to act but it failed to do so.”

San Bruno officials will now explore other ways to force PG&E to make the communications public. That could include requesting a court order to compel the utility to make the disclosures, but a decision hasn”t been made yet, Strottman said.

On Wednesday, a PUC judge ordered the utility to provide emails involving 10 company executives or consultants by Jan. 20. That order came in response to a request from TURN.

“The emails we have seen are just the tip of the iceberg,” said state Sen. Jerry Hill, whose San Mateo County district includes San Bruno. “The PUC needs to assure the public that they are being transparent in their dealings with PG&E. It appears to be business as usual at the PUC and PG&E.”

Also on Thursday, PG&E was hit with a $10.9 million fine for its role in causing a natural gas explosion in Carmel in March. The explosion blew up an unoccupied home. A federal grand jury has also been looking into the incident.